The present invention relates to an apparatus and method for use with a container for storing a substance, and in particular, where the substance stored is subject to evaporative losses or where the container is susceptible to corrosion.
Examples of evaporative substances which may be typically stored in such containers include hydrocarbon fuels (such as petrol), lubricating oil, distilled liquids, alcohol and sewerage.
Such liquids are, for example, generally stored in containers such as above ground and below ground fuel tanks, fuel tanks for vehicles (including land craft, aircraft and marine craft), fuel reservoir tanks, oil storage tanks, and wine fermenting vats and sewerage storage tanks.
Volatile liquids, such as fuels, are subject to evaporative losses when stored in storage tanks. Tanks storing such liquids are also subject to internal corrosion as are storage tanks storing lubricating oil. Moisture entering such storage tanks results in emulsion of the lubricating oil.
Evaporation of liquid stored in containers have several disadvantageous effects. Firstly, it results in filling and evaporative losses of the stored liquid. It may also result in internal corrosion of the container. Furthermore, contamination of the stored liquid may occur along with increased humidity levels in the container. Where the liquid is a flammable liquid, the evaporative effect may also increase fire hazards. Escape of the vapor may also present an environmental risk where the liquid is toxic or hazardous, as in the case of storage of fuels and sewerage.
Filling losses occur when the container is filled or refilled with liquid. For example, when the stored liquid is fuel, during the filling process, the liquid entering the container pushes out the existing fuel vapors which are then lost to the ambient atmosphere outside the container. In the case of a flammable liquid, there is a risk that the vapor will ignite resulting in an explosion.
Evaporation of the stored liquid also results in breathing losses. These occur when the stored liquid is heating up or cooling down. In the heating up process, the liquid expands and its volume increases. This results in the liquid vapor above the liquid in the container being pushed out of the container into the atmosphere. In the cooling down process, the liquid contracts and its volume decreases. This results in air being drawn into the container from the atmosphere. When liquid is discharged from the container, air is again drawn into the container and condensed vapor will settle on the walls of the container. This causes corrosion and the oxygen in the air in the container will oxidise the stored liquid.
Several prior art systems have sought to address the problems identified above.
One previous system provides a breathing valve for the container which merely delays the in or out breathing of the liquid in the container.
To overcome filling losses, a vapor return is used. Thus, when the container is filled with liquid, the increased pressure which occurs in the container pushes the vapor back to the reservoir from which the container is being filled. In the case where the container being filled is a petrol storage tank which is being filled by a tanker truck, the vapor is pushed back into the tanker truck reservoir. However, this system addresses losses which occur only in connection with the filling procedure. When liquid is discharged from the container, air is still drawn into the container from the atmosphere to take up the volume of liquid discharged. This incoming air results in corrosion inside the container.
Another system is known from U.S. Pat. No. 3,747,800. In this system, the container is provided with an inner tank constructed of a flexible material. The liquid is contained within the inner tank. The inner tank expands and contracts in accordance with the volume of liquid contained therein.
A further system is disclosed in WO 85/01035 (International Patent Application PCTIHU84/00044). The system disclosed in this specification uses a sheet element provided above the volatile substances stored in a storage tank.
In accordance with one aspect of the present invention there is provided an apparatus for use with a container for storing a substance comprising:
bladder means arranged to be located in the container and above the substance stored therein,
pipe means to deliver inflation medium to said bladder means,
pressure control means to supply inflation medium to said bladder means via said pipe means,
first pressure relief means to allow escape of inflation medium from said bladder means to relieve pressure from said bladder means, said first pressure relief means is set to a first selected pressure that is higher than the vapor pressure of the substance in the container, and
second pressure relief means to allow escape of vapor of the substance from the container to relieve pressure from the container, said second pressure relief means is set to a second selected pressure that is higher than said first selected pressure,
wherein said pressure control means is arranged to control supply of inflation medium to said bladder means to maintain the pressure in said bladder means above the vapor pressure of the substance in the container and said bladder means substantially fills the space above the substance in the container, said first pressure relief means is arranged to relieve pressure build-up in said bladder means to maintain the pressure in said bladder means at or below said first selected pressure, and said second pressure relief means is arranged to relieve pressure build-up in the container to maintain the pressure in the container at or below said second selected pressure.
Preferably, the apparatus further comprises means to selectively close off the container from said second pressure relief means to prevent escape of vapor of the substance from the container via said second pressure relief means and thereby prevent pressure relief from the container via said second pressure relief means when required.
Preferably, the apparatus further comprises means to selectively close off said bladder means from said first pressure relief means to prevent escape of inflation medium from said bladder means via said first pressure relief means and thereby prevent pressure relief from said bladder means via said first pressure relief means when required.
Preferably, said first selected pressure is up to substantially 5% higher than the vapor pressure of the substance in the container such that the pressure in said bladder means is maintained at a level up to substantially 5% higher than the vapor pressure of the substance in the container.
Preferably, said second selected pressure is up to substantially 20% higher than the vapor pressure of the substance in the container.
Preferably, the volume of said bladder means when inflated is substantially the same as the volume of the container.
Preferably, the substance in the container comprises a liquid.
Preferably, said means to selectively close off the container from said second pressure relief means comprises a first isolation valve.
Preferably, said means to selectively close off said bladder means from said first pressure relief means comprises a second isolation valve.
Preferably, said sensor means comprises a pressure gage.
Preferably, said sensor means comprises a temperature gage.
Preferably, access means is provided to access the interior of said bladder means.
Preferably, inlet means is provided to enable said container to be purged of air with a gas during installation of said bladder means into said container.
Preferably, the volume of said bladder means when inflated is substantially the same as the space remaining in said container when said container is filled with liquid.
Preferably, a cover is provided to close off an opening of said container, said cover provided with fittings to which said bladder means is attachable.
Preferably, said cover is provided with shroud means extending therefrom and arranged to be positioned within said container, said shroud means housing equipment located in said container.
Preferably, said shroud means is provided with vent openings in an upper region thereof.
Preferably, said shroud means comprises a tube attached at one of its ends to said cover.
Preferably, said bladder means comprises one or more individual bladders.
Preferably, the inflation medium comprises gas.
More preferably, the inflation medium comprises air.
Alternatively, the inflation medium comprises liquid.
As a further alternative, the inflation medium comprises a combination of gas (for example, air) and liquid.
In accordance with a second aspect of the present invention there is provided
a method for use with a container for storing a substance comprising
installing bladder means into the container,
delivering inflation medium to said bladder means,
controlling the supply of the inflation medium to said bladder means to maintain the pressure in said bladder means above the vapor pressure of the substance in the container, allowing escape of inflation medium from said bladder means to relieve pressure build-up in said bladder means to maintain the pressure in said bladder means at or below a first selected pressure, said first selected pressure being higher than the vapor pressure of the substance in the container,
allowing escape of vapor of the substance stored in the container to relieve pressure build-up in the container to maintain the pressure in the container at or below a second selected pressure, said second selected pressure being higher than said first selected pressure, and maintaining said bladder means in an inflated condition to substantially fill the space above the substance in the container.
Preferably, said first selected pressure is up to substantially 5% higher than the vapor pressure of the substance in the container and the method further comprises maintaining the pressure in said bladder means at a level up to substantially 5% higher than the vapor pressure of the substance in the container.
Preferably, said second selected pressure is up to substantially 20% higher than the vapor pressure of the substance in the container.
Preferably, the substance in the container comprises a liquid.
Preferably, the inflation medium comprises gas.
More preferably, the inflation medium comprises air.
Alternatively, the inflation medium comprises liquid.
As a further alternative, the inflation medium comprises a combination of gas (for example, air) and liquid.
The apparatus and method of the present invention act to reduce evaporative losses and/or reduce the internal corrosion of the container, depending on the nature of the substance being stored.
The apparatus and method of the present invention achieve this by preventing formation of vapor from the liquid in the container and by preventing ingress of air into the container.
At the same time, the apparatus and method of the present invention prevent humidification of the container, reduce fire risks by preventing escape of possibly flammable vapors which could ignite, and preventing escape of environmentally damaging or undesirable vapors. This latter problem may otherwise, for example, occur where the substance being stored is toxic or sewerage.
The apparatus and method of the present invention employ pressurisation of a bladder inside the container storing the substance, typically a liquid.